Silicon integrated circuits continue to often use aluminum as the conductive material for vertical and horizontal interconnects in a multi-level metallization structure despite an increased emphasis on copper metallization for very advanced circuitry. The aluminum is most often deposited by magnetron sputtering. However, as the aspect ratio of vertical interconnects continues to increase, a geometry generally unfavorable for sputtering, aluminum sputtering faces increased challenges. Nonetheless, relatively conventional DC magnetron sputter reactors continue to be favored because of their simplicity, low cost, and long usage.
As schematically illustrated in the cross sectional view of FIG. 1, a DC magnetron sputter reactor 10 includes a vacuum chamber 12 arranged generally symmetrically about a central axis 14. A vacuum pump system 16 pumps the chamber 12 to a very low base pressure in the range of 10−8 Torr. However, a gas source 18 connected to the chamber through a mass flow controller 20 supplies argon as a sputter working gas. The argon pressure inside the chamber 12 is typically held in the low milliTorr range. A pedestal 22 arranged about the central axis 14 holds a wafer 24 or other substrate to be sputter coated. An unillustrated clamp ring or electrostatic chuck may be used to hold the wafer 24 to the pedestal 22, which is usually temperature controlled. A shield 26 protects the chamber walls and the sides of the pedestal 22 from sputter deposition. A target 28 having a planar front face is arranged in opposition to the pedestal 22 and has no substantial effective portion extending closer to the pedestal. For aluminum sputtering, at least the front face of the target 28 facing the wafer 24 is composed of aluminum or an aluminum alloy having no more than 10 at % of one or more alloying elements in addition to elemental aluminum. The target 28 is vacuum sealed to the chamber 12 through an isolator 30.
A DC power supply 32 electrically biases the target 28 negatively with respect to the shield 28, if electrically grounded, or other chamber part to cause the argon sputter working gas to discharge into a plasma such that the positively charged argon ions are attracted to the negatively biased target 28 and sputter material from it. Some of the sputtered material ejected from the target 28 is deposited as a layer on the wafer 24. In reactive ion sputtering, a reactive gas such as nitrogen is additionally admitted to the chamber to cause the deposition of a metal compound such as metal nitride. In some applications for sputtering copper or refractory barrier metals, an RF power source 34, for example operating at 13.56 MHz although other RF frequencies may be used, biases an electrode 36 in the pedestal 22 through a capacitive coupling circuit 38. In other applications including sputtering aluminum, the RF biasing circuitry is conventionally omitted and the pedestal 22 is left electrically floating.
Dependent upon the application, the wafer 24 may need to be heated or cooled during sputter coating. A controllable power supply 40 may supply current to a resistive heater 42 embedded in the pedestal 22 to thereby heat the wafer 24. On the other hand, a controllable chiller 44 may circulate chilled water or other refrigerant to a cooling channel 46 formed in the pedestal 22. Although unillustrated, further thermal control is effected by the controllable supply of argon thermal transfer gas delivered to a convolute channel formed in the top surface of the pedestal electrode 36 to thermally couple the wafer 24 to the pedestal 22.
The diode DC magnetron sputter reactor for aluminum sputtering conventionally does not include a RF inductive or microwave source of energy significantly coupling energy into the plasma.
The sputtering rate can be greatly increased by placing a magnetron 50 in back of the target 28. The magnetron 50, which is an aspect of the present invention, can assume various shapes and forms. It may include pairs of magnetic poles 52, 54 of opposed vertical magnetic polarity and typically arranged in a ring shape to form a ringshaped region 56 of a high-density plasma (HDP) adjacent the front face of the target 28. The HDP region 56 results from the magnetic field extending horizontally between neighboring magnetic poles 52, 54 trapping electrons, thereby increasing the plasma density. The increased plasma density greatly increases the sputtering of the adjacent region of the target 28. The plasma density is further increased by the magnetron 50 having an encompassing area significantly smaller than the area of the target being scanned and sputtered, for example, less than 15%, which thereby concentrates the target power in the reduced area of the magnetron 50. To provide a more uniform target sputtering pattern, the ring-shape magnetron 50 is typically offset from the central axis 14. A motor 60 drives a rotary shaft 62 extending along the central axis 14 and fixed to a plate 64 supporting the magnetic poles 52, 54 to rotate the magnetron 50 about the central axis 14. Rotating the offset magnetron 50 produces an azimuthally uniform time-averaged magnetic field. If the magnetic poles 52, 54 are formed by respective rings of opposed cylindrical permanent magnets, the plate 64 is advantageously formed of a magnetic material to serve as a magnetic yoke.
Magnetrons of several different designs have been applied to reactors of the general design illustrated in FIG. 1. Tepman describes in U.S. Pat. No. 5,320,728 a magnetron that has a flattened kidney shape. For example, as illustrated in the plan view of FIG. 2, a kidney-shaped magnetron 70 includes an outer pole 72 of one magnetic polarity surrounding an inner pole 74 of the other magnetic polarity. The two poles 72, 74 are typically formed of continuous bands of a soft magnetic stainless steel acting as pole pieces and underlaid by a plurality of permanent magnets. A gap 76 of nearly constant width separates the two poles 72, 74 and has periphery with a flattened kidney shape. The gap 76 defines an annular band in which the magnetic field between the two poles 72, 74 is approximately horizontal adjacent the sputtering face of the target 28. The kidney-shaped magnetron 70 is relatively large compared to the target 28, for example, having an encompassing area within the inner periphery of the outer pole 74 of greater than 25% of the total used area of the target, that is, the area scanned by the magnetron 70 and thereby sputtered. The rotation center 14 of the magnetron 70 typically falls on or near the inner portion of the inner pole 74. Parker illustrates several variations of the kidney-shaped magnetron in U.S. Pat. No. 5,242,566.
More recently, a self-ionizing plasma (SIP) sputtering process has been developed primarily for use in copper sputtering, as has been described by Fu et al. in U.S. Pat. No. 6,306,265, incorporated herein by reference in its entirety. SIP sputtering relies upon high target power, high wafer biasing, and a relatively small unbalanced magnetron. The high target power and small magnetron produce a significant fraction of sputter atoms that are ionized, which the biased wafer accelerates and attracts deeply within narrow aspect-ratio holes. A typical SIP magnetron 80 is illustrated in schematic bottom plan view in FIG. 3, although other shapes are possible, including racetrack, circular, oval, and others. The SIP magnetron 80 includes a generally triangularly shaped outer pole 82 of one vertical magnetic polarity along the central axis 14. A curved side 84 of the outer pole 82 generally follows the adjacent outside periphery of the target 28. An apex 86 of the shaped outer pole 82 falls close to the rotational center 14 of the SIP magnetron 80. Typically, the rotational center 14 falls within the outer pole 82 or very close outwardly towards the curved side 84. The outer pole 82 surrounds a triangularly shaped inner pole 88 having a magnetic polarity opposite that of the outer pole 82 and separated from it by a nearly constant gap 90. The magnetic field produced between the two poles 82, 88 and extending horizontally in front creates the high-density plasma region 56 of FIG. 1 but a minimal central field-free core. An SIP magnetron is usually small, having an encompassing area within the inner periphery of the outer pole 84 of less than 20% of the used area of the target.
The SIP magnetron 80 is unbalanced in the meaning that the total magnetic intensity of the outer pole 82, that is, the magnetic flux integrated over the area of the outer pole 82, is substantially greater than that of the inner pole 82, for example by a factor of at least 150% and preferably 200% or 300%. Typically, the unbalance is achieved by placing beneath the two pole pieces acting as the inner and outer poles 82, 88 a different number of similarly constructed but oppositely oriented permanent cylindrical magnets, for example, of NdBFe. However, other structures have been proposed. The unbalance causes the unbalanced portion of the magnetic field to project from the magnetron 50 or 80 towards the wafer 24 of FIG. 1, thereby extending the plasma and guiding the ionized sputter atoms perpendicular to the wafer surface and deep into the deep via hole, particularly in a long-throw reactor.
Neither of the above sputtering methods seems adequate to fill aluminum into a high aspect-ratio via hole 100, illustrated in the cross-sectional view of FIG. 4, formed through an upper dielectric layer 102 overlying a conductive feature 104 in a lower dielectric layer 106. In advanced integrated circuits, the hole 100 may have an aspect ratio of its depth to width of four or more. A thin barrier layer 108 typically of Ti or TiN or a combination thereof is coated onto the sides of the via hole 100 before an aluminum layer 110 is sputter coated thereon. Preferably, the barrier layer 108 is removed from the bottom of the via hole 100 either by selective sputtering conditions or with a separate etching step, as is well known in the art.
A conventional aluminum sputter coating using the Tepman magnetron of FIG. 3 and an unbiased wafer, however produces a generally isotropic flux pattern of neutral sputter atoms unsuitable for filling high aspect-ratio holes. In particular, overhangs 112 tend to develop at the top corners of the via hole 100 and sidewall and bottom coverage is poor. In particular, the overhangs 112 may close the via hole 100 before the hole 100 is filled, thereby leaving a void in the aluminum fill. Such voids are almost impossible to remove and create great reliability problems.
One method of avoiding overhangs includes heating the wafer to a temperature of 300 to 500° C. or even higher during sputtering so that the aluminum reflows into the bottom of the via hole 100. Reflow however becomes increasingly ineffective with via holes 100 of increasing aspect ratio. Also, the reflowed aluminum does not wet well to uncoated surfaces of other materials. As a result, the aluminum tends to agglomerate within the via hole 100 rather than forming a smooth layer required for filling such narrow holes. One method of avoiding such agglomeration includes a two-step sputtering process in which a first sputter deposition step is performed with a relatively cool wafer so that the aluminum sticks to the oxide sidewalls and forms a thin first layer and a second sputter deposition is performed at a much higher temperature to flow over the first layer and fill the remaining portion of the hole. However, this technique practiced with conventional aluminum sputter reactors using a Tepman magnetron does not solve the problem of conformally coating the first layer into high aspect-ratio holes.
SIP sputtering is not typically used for aluminum sputtering. Even though it is likely to eliminate the overhangs and improve sidewall and bottom cover, it is felt that the small-size SIP magnetron creates significant radial non-uniformity in the thickness of a blanket portion 114 of the aluminum layer on the top surface of the dielectric layer 102. The blanket portion 114 is relatively thick so its deposition time needs to be minimized and its thickness made uniform for device reliability. SIP sputtering is considered insufficient for these objectives.
Accordingly, an aluminum sputtering deposition process is desired which can uniformly fill high aspect-ratio holes. Most preferably, the process would use only planar diode sputter reactors.